The present invention relates to a method of preparing sodium monosulfide by means of reacting sodium and sulfur under protective gas.
Sodium monosulfide, Na.sub.2 S, is an important reagent for introducing sulfur into organic molecules. Several methods are known according to which this compound can be prepared. It can be obtained e.g. by means of reacting sodium salts with hydrogen sulfide in aqueous or alcoholic solutions or by means of the reduction of sodium sulfate with carbon or hydrogen. The known methods share the common problem that products contaminated with reactants always accumulate which products must be separated from the impurities by being dissolved in suitable solvents and by filtration.
Since the elements sodium and sulfur react extremely vigorously with one another (enthalpy of formation for Na.sub.2 S: .DELTA.H.sub.B =-389.1 kJ/mole), a method of preparation proceeding directly from the elements which are commercially available in great purity could not be used for industrial purposes in the past.
Moisture-free Na.sub.2 S in sufficiently pure form was only obtainable by means of dewatering the hydrate Na.sub.2 S.multidot.9H.sub.2 O under an atmosphere of hydrogen. This hydrate had to be prepared by means of reacting sodium hydrogen sulfide with NaOH in a polar solvent (Kirk-Othmer, 3d edition (1982), vol. 18, pp. 793-847, especially pp. 803 and 809).
German patent application P 38 31 737.0 describes a method of preparing low sodium polysulfides and sodium in which the higher sodium polysulfide is placed in a receiver under protective gas in molten form and the sodium is charged into the molten melt in a stoichiometric proportion corresponding to the desired polysulfide product. Vigorous agitation is employed during the charging operation. Amounts are used such that the reaction mixture remains in the state of an agitatible suspension and the agitation is continued until the formation of the product has been completed. It would also be basically possible according to this known method to recover sodium monosulfide by providing the required stoichiometric proportion for the bound sulfur and the sodium. However, since the known method requires a melt as the reaction medium which consists at least in the final stage of the reaction of Na.sub.2 S alone, an economically feasible reactor material would hardly be available. This is so as a consequence of the very high melting point of this compound (1180.degree.-1200.degree. C.) and on account of the strong chemical aggressivity of the melt.
A method reported in Synth. React. Inorg. Met.-Org. Chem. 14 (7), pp. 945-951 (1984) is likewise directed to the recovery of lower-melting sodium polysulfides, this known method concerns a reaction of molten sodium placed in a receiver with added sulfur. However, in this prior method a molten intermediary product is obtained which is contaminated with sodium and substances of unknown composition. This product must then be ground to a powder after separation of the non-reacted sodium in a separate method step and further processed to the final product after reintroduction of the previously separated sodium and addition of more sulfur under renewed melting of the polysulfide produced.